OPA / Conftest Reference

Covers Rego v1 syntax, rule types, package namespacing, input shapes, unit testing, Conftest CLI, Regal linting, formatting, GitHub Actions integration, and troubleshooting.

Rego v1 Syntax

Always include import rego.v1 at the top of every policy file. This enables modern syntax (if, contains, in, every) and disables Rego v0 quirks.

# METADATA
# title: Deny public S3 buckets
# description: S3 buckets must not be publicly accessible
# authors:
# - Platform Team <platform@example.com>
# entrypoint: true
package terraform.s3

import rego.v1

Core constructs

# Iteration with some
deny contains msg if {
    some resource_name
    resource := input.resource.aws_s3_bucket[resource_name]
    resource.acl == "public-read"
    msg := sprintf("S3 bucket '%s' must not have public-read ACL", [resource_name])
}

# Set membership with in
allowed_regions := {"eu-central-1", "eu-west-1"}

deny contains msg if {
    some name
    resource := input.resource.aws_instance[name]
    not resource.region in allowed_regions
    msg := sprintf("EC2 instance '%s' must be in an allowed region", [name])
}

# String helpers
deny contains msg if {
    some name
    image := input.resource.aws_ecr_repository[name]
    not startswith(image.name, "prod-")
    msg := sprintf("ECR repository '%s' name must start with 'prod-'", [name])
}

# Negation — not (AWS provider >= 5: encryption is a separate resource)
deny contains msg if {
    some name
    _ = input.resource.aws_s3_bucket[name]
    not _has_enc_resource(name)
    msg := sprintf("S3 bucket '%s' must have an aws_s3_bucket_server_side_encryption_configuration resource", [name])
}

_has_enc_resource(bucket_name) if {
    some enc_name
    enc := input.resource.aws_s3_bucket_server_side_encryption_configuration[enc_name]
    enc.bucket == bucket_name
}

# Existential check — deny if any ingress rule is fully open
deny contains msg if {
    some name
    group := input.resource.aws_security_group[name]
    some rule in group.ingress
    rule.from_port == 0
    rule.to_port == 65535
    msg := sprintf("Security group '%s' has an overly permissive ingress rule", [name])
}

Rule Types

Rule	Conftest behaviour	When to use
`deny`	Fails the check (exit code 1)	Hard requirements — must not be violated
`warn`	Prints warning but passes	Advisory — should be fixed but not blocking
`violation`	Fails (used by Gatekeeper / OPA policy sets)	Framework integrations

Rules must be named starting with deny, warn, or violation — Conftest silently ignores any other rule name.

Use set comprehensions (deny contains msg if { ... }) rather than booleans so Conftest can report the message string.

METADATA Block

Every policy file should have a METADATA block immediately before the package declaration:

# METADATA
# title: Short human-readable title
# description: One-sentence description of what this policy enforces
# authors:
# - Team Name <email@example.com>
# organizations:
# - Your Org
# entrypoint: true
package my.namespace

entrypoint: true marks the package as an OPA bundle entrypoint for tools like opa build and Regal's no-defined-entrypoint rule. Conftest discovers policies by package name and rule names (deny/warn/violation) — not by this flag. Include it for lint compliance and bundle compatibility.

Package Namespacing

package main — default namespace; used when running conftest test without --namespace
Named packages — use for multi-domain repos to avoid collisions

package terraform.iam      # --namespace terraform.iam
package k8s.pods           # --namespace k8s.pods
package github.repository  # --namespace github.repository

Run a specific namespace:

conftest test --policy ./policies --namespace terraform.iam plan.json

Run all namespaces:

conftest test --policy ./policies --all-namespaces plan.json

Input Shape

Conftest parses config files into a JSON input object. The shape depends on the file type.

Terraform HCL

# main.tf
resource "aws_s3_bucket" "my_bucket" {
  bucket = "my-app-data"
  acl    = "private"
}

Parsed as:

{
  "resource": {
    "aws_s3_bucket": {
      "my_bucket": {
        "bucket": "my-app-data",
        "acl": "private"
      }
    }
  }
}

Access in Rego:

input.resource.aws_s3_bucket[name].acl

Terraform Plan JSON

terraform plan -out=tfplan.binary
terraform show -json tfplan.binary > tfplan.json
conftest test --policy ./policies tfplan.json

The plan JSON has a different shape — use input.resource_changes[_]:

deny contains msg if {
    some change in input.resource_changes
    change.type == "aws_s3_bucket"
    change.change.after.acl == "public-read"
    msg := sprintf("S3 bucket '%s' must not have public-read ACL", [change.name])
}

Kubernetes YAML

apiVersion: apps/v1
kind: Deployment
metadata:
  name: my-app
spec:
  template:
    spec:
      containers:
        - name: app
          image: my-app:latest

Parsed as:

{
  "apiVersion": "apps/v1",
  "kind": "Deployment",
  "metadata": { "name": "my-app" },
  "spec": {
    "template": {
      "spec": {
        "containers": [{ "name": "app", "image": "my-app:latest" }]
      }
    }
  }
}

Inspect actual input

Always check the actual parsed input before writing rules:

conftest parse main.tf
conftest parse deployment.yaml
conftest parse tfplan.json

Writing Policies

Full example — Terraform IAM policy

# METADATA
# title: IAM policy least privilege
# description: IAM policies must not use wildcard actions or resources
# authors:
# - Platform Team <platform@example.com>
# entrypoint: true
package terraform.iam

import rego.v1

deny contains msg if {
    some name
    policy := input.resource.aws_iam_policy[name]
    some statement in policy.policy.Statement
    statement.Effect == "Allow"
    statement.Action == "*"
    msg := sprintf("IAM policy '%s' must not use wildcard Action '*'", [name])
}

deny contains msg if {
    some name
    policy := input.resource.aws_iam_policy[name]
    some statement in policy.policy.Statement
    statement.Effect == "Allow"
    statement.Resource == "*"
    statement.Action != "*"
    msg := sprintf("IAM policy '%s' must not use wildcard Resource '*'", [name])
}

warn contains msg if {
    some name
    role := input.resource.aws_iam_role[name]
    not role.description
    msg := sprintf("IAM role '%s' should have a description", [name])
}

Full example — Kubernetes pod security

# METADATA
# title: Pod security baseline
# description: Pods must run as non-root with read-only root filesystem
# authors:
# - Platform Team <platform@example.com>
# entrypoint: true
package k8s.pods

import rego.v1

deny contains msg if {
    input.kind == "Deployment"
    some container in input.spec.template.spec.containers
    not container.securityContext.runAsNonRoot
    msg := sprintf("Container '%s' must set runAsNonRoot: true", [container.name])
}

deny contains msg if {
    input.kind == "Deployment"
    some container in input.spec.template.spec.containers
    not container.securityContext.readOnlyRootFilesystem
    msg := sprintf("Container '%s' must set readOnlyRootFilesystem: true", [container.name])
}

deny contains msg if {
    input.kind == "Deployment"
    some container in input.spec.template.spec.containers
    container.image == _
    endswith(container.image, ":latest")
    msg := sprintf("Container '%s' must not use ':latest' image tag", [container.name])
}

Unit Tests

Test files must be named <policy>_test.rego and placed in the same directory.

# METADATA
# title: Tests for IAM policy least privilege
package terraform.iam_test

import data.terraform.iam
import rego.v1

# --- deny wildcard action ---

test_deny_wildcard_action if {
    input := {
        "resource": {
            "aws_iam_policy": {
                "my_policy": {
                    "policy": {
                        "Statement": [{"Effect": "Allow", "Action": "*", "Resource": "arn:aws:s3:::*"}]
                    }
                }
            }
        }
    }
    count(iam.deny) == 1 with input as input
}

test_allow_scoped_action if {
    input := {
        "resource": {
            "aws_iam_policy": {
                "my_policy": {
                    "policy": {
                        "Statement": [{"Effect": "Allow", "Action": "s3:GetObject", "Resource": "arn:aws:s3:::my-bucket/*"}]
                    }
                }
            }
        }
    }
    count(iam.deny) == 0 with input as input
}

# Helper function for building fixtures
make_policy(action, resource) := {
    "resource": {
        "aws_iam_policy": {
            "test_policy": {
                "policy": {
                    "Statement": [{"Effect": "Allow", "Action": action, "Resource": resource}]
                }
            }
        }
    }
}

test_deny_wildcard_resource if {
    count(iam.deny) == 1 with input as make_policy("s3:GetObject", "*")
}

Run tests:

conftest verify --policy ./policies

Validation Pipeline

Run these in order — fix each before proceeding to the next.

1. Format check

# Check — non-zero exit if any file is unformatted
conftest fmt ./policies/*.rego --check

# Auto-fix — rewrites files in place
conftest fmt ./policies/*.rego

2. Regal lint

# Install
brew install styrainc/packages/regal   # macOS
# or
curl -L -o regal "https://github.com/StyraInc/regal/releases/latest/download/regal_$(uname -s)_$(uname -m)"
chmod +x regal && sudo mv regal /usr/local/bin/

# Lint all .rego files
regal lint ./policies

Configure Regal with .regal/config.yaml:

# .regal/config.yaml
rules:
  idiomatic:
    no-defined-entrypoint:
      level: error
  style:
    line-length:
      level: warning
      max-line-length: 120
    prefer-snake-case:
      level: warning
  bugs:
    rule-named-if:
      level: error
    unused-variable:
      level: error

3. Unit tests

conftest verify --policy ./policies

4. Integration test against real input

# HCL files
conftest test --policy ./policies ./terraform/*.tf

# Terraform plan
conftest test --policy ./policies ./tfplan.json

# Kubernetes manifests
conftest test --policy ./policies ./k8s/*.yaml --all-namespaces

# From a remote git repo (pulls policies then tests)
conftest test \
  --update "git::https://github.com/your-org/opa-policies.git//terraform" \
  --all-namespaces \
  ./terraform/*.tf

Terraform CI Pipeline Placement

For Terraform policies, conftest runs after terraform validate and before terraform plan as a blocking gate. If conftest fails, the plan step must not run.

- run: terraform validate
- run: conftest test --policy ./policies ./main.tf   # AFTER validate, BEFORE plan
- run: terraform plan -out=tfplan.binary

This ordering ensures:

terraform validate confirms HCL syntax and provider schema
conftest test enforces policy rules (IAM wildcards, tagging, encryption) on the source — before a plan is generated
terraform plan only runs if both validate and policy gates pass

For plan-level analysis (checking what Terraform will do), run conftest a second time on tfplan.json alongside plan:

terraform show -json tfplan.binary > tfplan.json
conftest test --policy ./policies tfplan.json

GitHub Actions Integration

name: Policy validation

on:
  pull_request:

jobs:
  opa:
    name: OPA / Conftest
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@b4ffde65f46336ab88eb53be808477a3936bae11  # v4.1.1

      - name: Install Conftest
        run: |
          VERSION=$(curl -s https://api.github.com/repos/open-policy-agent/conftest/releases/latest | jq -r .tag_name)
          curl -Lo conftest.tar.gz "https://github.com/open-policy-agent/conftest/releases/download/${VERSION}/conftest_${VERSION#v}_Linux_x86_64.tar.gz"
          tar xzf conftest.tar.gz conftest
          sudo mv conftest /usr/local/bin/

      - name: Install Regal
        run: |
          VERSION=$(curl -s https://api.github.com/repos/StyraInc/regal/releases/latest | jq -r .tag_name)
          curl -Lo regal "https://github.com/StyraInc/regal/releases/download/${VERSION}/regal_Linux_x86_64"
          chmod +x regal && sudo mv regal /usr/local/bin/

      - name: Check formatting
        run: conftest fmt ./policies --check

      - name: Lint with Regal
        run: regal lint ./policies

      - name: Run unit tests
        run: conftest verify --policy ./policies

      - name: Run integration tests
        run: conftest test --policy ./policies --all-namespaces ./tests/**/*.tf ./tests/**/*.yaml

Shared Data (allow-lists)

Use data.* for allow-lists shared across policies — store in a JSON or YAML file alongside the policies.

// policies/data/allowed_regions.json
{
  "allowed_regions": ["eu-central-1", "eu-west-1", "us-east-1"]
}

import rego.v1

deny contains msg if {
    some name
    instance := input.resource.aws_instance[name]
    not instance.region in data.allowed_regions
    msg := sprintf("Instance '%s' is not in an allowed region", [name])
}

Troubleshooting

Symptom	Cause	Fix
Policy produces no output, input matches	Rule not named `deny`/`warn`/`violation`	Rename rule to start with `deny`, `warn`, or `violation`
`conftest test` passes but shouldn't	Namespace mismatch	Run with `--all-namespaces` or add `--namespace <package>`
`undefined ref: input.X`	Input shape wrong	Run `conftest parse <file>` and check actual field path
Rule fires on everything	Missing `some` for iteration	Add `some <name>` before iterating over `input.resource[name]`
`import rego.v1` causes error	Old Conftest version	Upgrade to Conftest >= 0.46.0
`conftest verify` finds no tests	Test file not `_test.rego`	Rename to `<policy>_test.rego`
Regal: `no-defined-entrypoint`	Missing `# entrypoint: true` in METADATA	Add METADATA block with `entrypoint: true`
Boolean rule always true/false	Using boolean instead of set comprehension	Change `deny if { ... }` to `deny contains msg if { ... msg := "..." }`
`conftest fmt --check` fails	File not canonically formatted	Run `conftest fmt ./policies/*.rego` to auto-fix

Rego v1 Syntax​

Core constructs​

Rule Types​

METADATA Block​

Package Namespacing​

Input Shape​

Terraform HCL​

Terraform Plan JSON​

Kubernetes YAML​

Inspect actual input​

Writing Policies​

Full example — Terraform IAM policy​

Full example — Kubernetes pod security​

Unit Tests​

Validation Pipeline​

1. Format check​

2. Regal lint​

3. Unit tests​

4. Integration test against real input​

Terraform CI Pipeline Placement​

GitHub Actions Integration​

Shared Data (allow-lists)​

Troubleshooting​